The invention relates generally to acoustic resonators and more particularly to tailoring the filter response for a passband filter having film bulk acoustic resonators.
In different communications systems, the same signal path functions as both an input to a receiver and an output from a transmitter. For example, in a cellular or cordless telephone, an antenna may be coupled to the receiver and to the transmitter. In such an arrangement, a duplexer is often used to couple the common signal path to the input and to the output. The function of the duplexer is to provide the necessary coupling to and from the common signal path, while preventing the signals generated by the transmitter from being coupled to the input of the receiver.
One type of duplexer is referred to as a xe2x80x9cfull duplexer.xe2x80x9d A full duplexer operates properly only if the transmit signal is carried at a frequency that is different than the frequency of the receive signal. The full duplexer utilizes passband filters that isolate the transmit signal from the receive signal according to the frequencies. FIG. 1 illustrates a conventional circuit used in cellular telephones, personal communication system (PCS) devices and other transmit/receive devices. A power amplifier 10 of a transmitter is connected to a transmit port 12 of a full duplexer 14. The duplexer also includes a receive port 16 that is connected to a low noise amplifier (LNA) 18 of a receiver. In addition to the transmit port and the receive port, the duplexer includes an antenna port 20 which is connected to an antenna 22.
The duplexer 14 employs a transmit passband filter 24, a receive passband filter 26, and a phase shifter 28. The passbands of the two filters 24 and 26 are respectively centered on the frequency range of the transmit signal from the transmit port 12 and the receive signal to which the receiver is tuned.
The requirements of the passband filters 24 and 26 of the duplexer 14 are stringent. The passband filters must isolate low intensity receive signals generated by the antenna for input to the low noise amplifier 18 from the strong transmit signals generated by the power amplifier 10. In a typical embodiment, the sensitivity of the low noise amplifier may be in the order of xe2x88x92100 dBm, while the power amplifier may provide transmit signals having an intensity of approximately 28 dBm. The duplexer 14 must attenuate the transmit signal by approximately 50 dB between the antenna port 20 and the receive port 16 to prevent any residual transmit signal that may be mixed with the receive signal from overloading the low noise amplifier 18.
One standard for use in PCS devices for a mobile telephone is the code division multiple access (CDMA) standard. A CDMA 1900 MHz mobile phone has a transmit filter 24 with a passband of 1850 MHz to 1910 MHz and has a receive filter 26 with a passband of 1930 MHz to 1990 MHz. A filter response 30 for the transmit filter is shown in FIG. 2. The filter response is defined by poles and zeros (i.e., nulls) of acoustic resonators. The poles and zeros are equidistantly spaced from a center frequency 32. During ideal conditions, the attenuation within the range of frequencies from 1850 MHz to 1910 MHz is relatively small. That is, the filter response 30 exhibits a relatively small insertion loss. On the other hand, the attenuation beyond the target passband is substantial. As shown in FIG. 2, there is a steep roll-off at both the high frequency end and the low frequency end of the filter response. The steep roll-off at the high frequency end ensures isolation from the passband of the receive filter 26, which is only 20 MHz above the passband of the transmit filter.
There are a number of available approaches to fabricating a duplexer. The conventional approach is to use ceramic technology. That is, ceramic-based half-wave and quarter-wave resonators are fabricated and connected to provide the poles and zeros which define the desired filter response. A significant built-in advantage of ceramic filters is that the temperature coefficient of such a filter is close to zero. Thus, the filter response does not materially change in shape or location as a result of temperature variations.
One concern with the use of ceramic duplexers is that there is a relationship between the quality factor xe2x80x9cQxe2x80x9d of the filter and the size of the filter. For a ceramic filter, Q decreases with the decreasing size of the filter. In applications such as the CDMA market, the guard band between the transmit passband and the receive passband is very narrow (20 MHz). Since Q affects the steepness of the roll-off of the filter response, the Q must remain within a set range if the roll-off of the filter response is to meet the specifications set forth by the requirements of the system. Therefore, the duplexer that is fabricated using ceramic technology has a certain minimum volume that is relatively large. In fact, of the components of a CDMA 1900 MHz telephone, only the battery is larger than a ceramic-based duplexer.
Alternative approaches to using ceramic-based duplexers include fabricating surface acoustic wave (SAW) duplexers or film bulk acoustic resonator (FBAR) duplexers. Both of these types of duplexers occupy much smaller volumes than the ceramic duplexers, since the limiting factors for the Q are governed by the properties of sound waves, rather than electrical resistance. A typical SAW or FBAR die size (e.g., silicon chip size) is on the order of 0.25 mm. The height is governed by the die package requirements, but can be made under 2 mm. A drawback for both SAW and FBAR duplexers is that both technologies suffer from frequency shifts as a result of temperature variations. As the duplexer increases in temperature, the stiffness of the resonating materials decreases. The decrease in material stiffness results in a shift in the sound wave velocity, since the sound velocity is dependent upon the square root of the mass density divided by the stiffness. It follows that the filter response shifts downwardly in frequency as the temperature rises. SAW duplexers also have problems with power handling capabilities and achieving a relatively high Q. It has not yet been shown that SAW duplexers can meet the performance requirements for use in CDMA 1900 MHz telephones.
FBAR technology has three advantages over SAW technology. First, FBAR duplexers have been shown to have excellent power handling abilities. Second, FBAR resonators demonstrate Qs that are significantly higher than those identified in publications regarding SAW resonators. Using FBAR resonators, it is possible to achieve a 10.5 MHz roll-off (from 3.3 dB to 47.5 dB) for the transmitter portion of a CDMA PCS duplexer. In comparison, ceramic duplexers have approximately a 20 MHz roll-off. The third advantage of FBAR duplexers over SAW duplexers is that they tend to have a lower temperature coefficient. SAW resonators made from lithium niobate have a frequency shift of approximately 90 ppm/xc2x0 C., and SAW resonators made with lithium tantalate have a frequency shift of approximately 34 ppm/xc2x0 C. In comparison, FBAR duplexers have been measured to have a frequency shift between 20 and 30 ppm/xc2x0 C.
As previously noted, within the CDMA PCS specification, there is a 20 MHz guard band between the transmitter and receiver passbands. The goal of a duplexer is to allow as much energy through each passband, while rejecting nearly all energies outside of the passband. If a realistic FBAR duplexer has a 50 dB roll-off in 10 MHz, this leaves 10 MHz for process variation and temperature shift. In percentage terms, this is slightly greater than 0.5 percent (i.e., 10 MHz/1920 MHz). If it is assumed that an FBAR filter has a temperature-dependent frequency shift of 30 ppm/xc2x0 C., and it is assumed that system requirements must meet specifications over a temperature range of xe2x88x9220xc2x0 C. to 60xc2x0 C., the total temperature shift may be as great as 4.8 MHz. Additionally, heating of the FBAR filter as a result of absorption of input power may extend the potential frequency shift to 6 MHz. Using 10 MHz for the roll-off and 6 MHz for the temperature-induced shift, there are only 4 MHz that remain for process variations within the fabrication procedure. Moreover, there are back-end variations in assembly which may affect the tolerances.
What is needed is a filter and a method of fabricating the filter which mitigate the adverse effects of temperature variations.
A filter includes an array of acoustic resonators that cooperate to establish an asymmetrical filter response over a target passband of frequencies. In the preferred embodiment, the acoustic resonators are film bulk acoustic resonators (FBARs) that include series FBARs coupled in electrical series and at least one shunt FBAR that is connected between adjacent series FBARs. The series FBARs determine the response characteristics at one end of a filter response over the target passband, while the shunt FBAR or FBARs determine the response characteristics at the opposite end. In the most preferred embodiment, the filter is a transmitter portion of a duplexer, so that the series FBARs determine the response characteristics at the high frequency end. In this embodiment, the passband of the filter has an insertion loss profile in which a minimum insertion loss is located at or near the high frequency end of the filter response and a greater insertion loss is exhibited at the low frequency end.
The passband of the filter is determined by the resonant frequencies of the acoustic resonators. Poles and zeros are selected to tailor the filter response. In the preferred embodiment, the insertion loss profile of the filter response over the target passband progressively declines from the minimum insertion loss located at or near the high frequency end of the target passband to a maximum insertion loss located at or near the low frequency end. That is, there is a filter response slope within the target passband. Furthermore, there preferably is a steep roll-off adjacent to the high frequency end of the target passband and a gradual roll-off adjacent to the low frequency end. The tailoring of the filter response can be accomplished using known methods, such as by selectively adjusting the effective coupling coefficients of the FBARs or by selectively adjusting the impedances of the FBARs. As another alternative, auxiliary inductances may be intentionally introduced into electrical series with one or more of the FBARs, so as to slightly reduce the resonant frequency. For example, all of the shunt FBARs of a transmitter filter may be fabricated to have the same resonant frequency, but one shunt FBAR may be connected to an auxiliary inductor that changes the characteristics of the transmitter filter at the low frequency end of the passband.
The goal in the tailoring of the filter response is to address the xe2x80x9cworst casexe2x80x9d scenario for operation of the system in which the filter resides. The tailoring sacrifices performance at the duplexer portion of the system in order to compensate for weaknesses at the power amplification portion. This xe2x80x9cworst casexe2x80x9d scenario occurs when the ambient temperature and the power requirements are simultaneously high. The elevated temperature tends to negatively affect the efficiency of the power amplifier. When the efficiency of the power amplifier decreases as a result of a temperature increase, the operation of the power amplifier is automatically adjusted by signaling from the base station to compensate for the loss in radio frequency (rf) power. The resulting power boost requires higher amounts of heat to be dissipated as a result of the increased dc losses.
Another factor of the xe2x80x9cworst casexe2x80x9d scenario is that as the FBAR filter begins to heat and the filter response shifts downwardly in frequency, the power absorbed by the filter increases dramatically. This increases the temperature of the filter. At higher temperatures, a given filter will experience more insertion loss across its entire passband. The higher insertion loss causes more power to be dissipated in the filter. All of these factors contribute to a xe2x80x9cpositive feedbackxe2x80x9d effect. The signal that is transmitted to the antenna will degrade quickly as conditions approach the xe2x80x9cworst casexe2x80x9d scenario.
By tailoring the filter response in the manner described above, a xe2x80x9chumpxe2x80x9d is formed at the high frequency end of the transmitter portion of a duplexer. There is a reduction in filter performance at the low frequency end as a result of the tailoring at the high frequency end, but the overall system performance is enhanced. As ambient temperatures rise and power from the power amplifier increases (increasing both circuit board temperature and locally elevating the filter temperature via additional power absorbed at the filter), the overall insertion loss of the filter remains substantially constant.
The description of the xe2x80x9cworst casexe2x80x9d scenario relates only to the high frequency end of the filter response of the transmitter portion. The present invention recognizes that the adverse effects exhibited at the low frequency end of the passband are significantly less severe. If the ambient temperature drops from room temperature to xe2x88x9220xc2x0 C., the insertion loss will be less than what it would be for a filter having a symmetrical filter response. However, there are two mitigating factors that create a xe2x80x9cnegative feedbackxe2x80x9d effect (as opposed to the positive feedback effect described above). The first mitigating factor is that as the temperature decreases, the overall insertion loss reduces, since electrical losses and thermo-acoustic scattering losses decrease. The second mitigating factor is that the power amplifier is xe2x80x9cslavedxe2x80x9d to the PCS base station. If the base station perceives that the power amplifier is not generating sufficient power, it will instruct the remote PCS device to increase the power output of the power amplifier. At lower temperatures, the power amplifier is more efficient and can comply with the request of the base station without dissipating a great amount of power. With the increase in power, the filter is heated by the added power directly in the FBAR filter and by the residual increase in heat emanated from the power amplifier.
The strategy of designing the asymmetrical filter response allows designers to overcome a major problem of performance degradation at elevated temperatures. Such a design is particularly useful in applications in which there are extremely tight tolerances and error budgets, such as those associated with the 1900 MHz PCS band. However, the asymmetrical filter response carries benefits in other applications.